if (document.addEventListener) document.addEventListener('keypress', F6IptalEt, true);
else if (document.attachEvent) document.attachEvent('onkeydown', F6IptalEt);
else document.onkeydown = F6IptalEt;

function F6IptalEt(event) {
    if (!event) event = window.event;
    if (!event) return;

    var keyCode = event.keyCode ? event.keyCode : event.charCode;
   
    //117 F6 tusu
    if (keyCode == 117) {        

        // Standart DOM (Mozilla):
        if (event.preventDefault) event.preventDefault();

        //IE 
        if (document.all && window.event && !event.preventDefault) {
            event.cancelBubble = true;
            event.returnValue = false;
            event.keyCode = 0;
        }
        alert('F6 Tusu Malesef Yasak');
        return false;
    }
}